Magnetic recording media generally comprise a magnetic layer coated onto at least one side of a nonmagnetizable substrate. For particulate magnetic recording media, the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetic layer may also include other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
Some forms of magnetic recording media, such as magnetic recording tape, may also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating also includes a polymeric binder and other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
The polymeric binders of the magnetic layer and the backside coating are commonly derived from polymers which require curing in order to provide magnetic recording media with appropriate physical and electromagnetic properties. To prepare such media, the components of the magnetic layer or the backside coating, as appropriate, are combined with a suitable solvent and thoroughly mixed to form a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the coating is dried, calendered if desired, and then cured.
The polymeric binders of magnetic recording media are most commonly prepared from polymer blends comprising a hard resin component, i.e., a polymer with relatively high glass transition temperature (T.sub.g) and modulus, and a soft resin component, i.e. a polymer with relatively low glass transition temperature and modulus. In the past, polyurethane polymers have been widely used as the soft resin component.
Copolymers based on vinyl chloride or vinylidene chloride have been widely used as the hard component of choice for use with polyurethanes, due to their miscibility and functional compatibility with polyurethanes and their relatively high glass transition temperatures, moduli, and hardnesses. Vinyl chloride or vinylidene chloride copolymers, however, tend to degrade over time, releasing gaseous HCl which can change the properties of the media as well as corrode the recording head or the like.
Accordingly, some investigators have described vinyl copolymers used in magnetic recording media, wherein the use of vinyl chloride and vinylidene chloride has been avoided. See, e.g., U.S. Pat. Nos. 5,098,783; 4,876,149; and 4,837,082; and Japanese Publication Nos. SHO 62-30162; SHO 54-84708; SHO 54-46519; and SHO 54-46518.
It is extremely desirable to lubricate the magnetic layer in order to minimize friction between the magnetic layer and the recording/playback head used to transfer signals to and from the magnetic layer. According to one approach for lubricating the magnetic layer, low molecular weight lubricant additives are blended into the dispersion used to form the magnetic layer. However, lubricants internal to the dispersion are not always desirable. Such lubricants may tend to plasticize the polymeric binder and thereby decrease the modulus of the magnetic layer. Further, excess lubricant can exude from the magnetic layer over time, leading to changes in the properties of the magnetic layer as well as to contamination of the head.
In another approach, topical lubricants having a low surface energy such as fluorinated monomers and oligomers may be applied to the surface of a magnetic recording medium such as a rigid disk. Topical lubricants are not generally used for flexible magnetic recording media such as high density diskettes because of the difficulty of providing a thin, uniform lubricant coating on the surface of such media. Incorporation of such fluorinated materials into the dispersion for the magnetic layer of a flexible medium is likewise unsuitable because of the poor solubility of the fluorinated material in common magnetic coating solvents such as methyl ethyl ketone (MEK).
What is needed in the art is a hard resin component of a polymeric binder which has no vinyl chloride or vinylidene chloride components, is soluble in common magnetic coating solvents, and is compatible with other magnetic layer and backside coating components. What is also needed in the art is a way to lubricate magnetic layers without relying solely upon low molecular weight lubricant additives.